1. Technical Field of the Invention
The present invention relates generally to an ignition system for internal combustion engines, and more particularly to a so-called induction discharge non-contact type ignition system which interrupts a primary current flow through an ignition coil to produce a desired level of required secondary voltage for initiating an ignition arc through a spark plug.
2. Background Art
U.S. Pat. No. 5,193,514 to Kobayashi et al. and U.S. Pat. No. 3,824,977 to Campbell et al. teach conventional ignition systems for internal combustion engines.
FIG. 9 shows a prior art ignition system similar to those disclosed in the above references. An igniter 20 has a Darlington transistor 21 serving to turn on and off a primary current flowing through an ignition coil 22. The Darlington transistor 21 is provided with two npn bipolar transistors.
The ignition coil 22 has a primary winding 23 connected to a collector electrode of the Darlington transistor 21 and a secondary winding 24 connected to a spark plug (not shown). A zener diode 25 is connected to the Darlington transistor 21 for protecting the transistor 21 against the overvoltage. A breakdown voltage V.sub.D of the transistor 21 is determined based on a zener voltage V.sub.Z of the zener diode 25 which is selected to be about 350 V in view of effective withstand voltage characteristics of the Darlington transistor 21.
In recent years, a DLI (Distributor Less Ignition) system which is designed to supply sparking energy of an ignition coil directly to a spark plug without use of a distributor, has become more prevalent. The DLI system, as shown in FIG. 10, has a high withstand voltage diode 100 interposed between a primary winding 8 and a secondary winding 9 of an ignition coil 7 for preventing the occurrence of a sparking failure of a spark plug due to a secondary on-voltage induced in the secondary winding 9 when a primary current is supplied to the ignition coil. The secondary on-voltage depends upon a secondary winding to primary winding turns ratio of the ignition coil.
It is difficult to decrease the turns ratio of the ignition coil to eliminate the need for the high withstand voltage diode 100 because in doing so, a secondary voltage would not reach a required level with a typical zener voltage V.sub.Z of 350 V.
The ignition system taught in U.S. Pat. No. 3,824,977 has a secondary winding to primary winding turns ratio of an ignition coil raging from 40 to 60 for increasing an ignition arc current produced through a spark plug to improve the sparking ability. Additionally, in the ignition system disclosed in U.S. Pat. No. 5,193,514, an ignition coil has a turns ratio of less than 70 for developing a voltage of at least 6 kV across electrodes of a spark plug. It will be noted that while conventional ignition coils commonly have a turns ratio of about 90, the ignition coils, as taught in the above references, have decreased turns ratios. These references, however, do not refer to a reduction in primary voltage of the ignition coil at all.
Generally, although a decrease in turns ratio of an ignition coil will produce various beneficial results, it becomes difficult to decrease the turns ratio as a required secondary voltage is increased.
In the prior art ignition system shown in FIG. 9, a primary voltage V.sub.1 of the primary winding 23 of the ignition coil 22 is determined based on a secondary voltage V.sub.2 and a turns ratio a according to the relation of V.sub.1 =V.sub.2 /a. Thus, a decrease in the turns ratio a to obtain a preselected level of the secondary voltage V.sub.2 will cause the primary voltage V.sub.1 to increase. When the primary voltage V.sub.1 exceeds the zener voltage V.sub.Z (=350 V), it is restricted by the zener voltage V.sub.Z so that the secondary voltage V.sub.2 of the ignition coil cannot reach a required secondary voltage V.sub.r.
Taking as an example a case where the turns ratio a is 70, if the required secondary voltage V.sub.r is relatively low (e.g., V.sub.r =15 kV), then the primary voltage V.sub.1 will be 15 kV/70=214 V. In this case, since V.sub.1 &lt;V.sub.Z (350 V), the primary voltage V.sub.1 is not affected by the zener voltage V.sub.Z so that the required secondary voltage V.sub.r is produced.
On the other hand, when the required secondary voltage V.sub.r is relatively high (e.g., V.sub.r =30 kV), then the primary voltage V.sub.1 will be 30 kV/70=428 V so that V.sub.1 &gt;V.sub.Z (350 V). The primary voltage V.sub.1 is, thus, affected by the zener voltage V.sub.Z so that it is increased only to 350 V. Accordingly, only a secondary voltage V.sub.2 of approximately 24.5 kV(=350 V.multidot.70) will be produced. The great decrease in the secondary voltage V.sub.2 relative to the required secondary voltage V.sub.r (=30 kV) increases the possibility of misfiring, thereby degrading the drivability. It will, thus, be appreciated that the use of a zener diode for protecting a switching element from the overvoltage to decrease the turns ratio, prohibits the secondary voltage V.sub.2 from being increased up to the required secondary voltage V.sub.r.
Additionally, the required secondary voltage V.sub.r usually tends to be increased with the passing of time, thereby increasing the possibility of the above problems being encountered. Further, in recent years, a compression ratio of an internal combustion engine is often increased for producing high power and/or an air-fuel ratio is often controlled on a lean side for fuel economy. This will, however, cause the required secondary voltage V.sub.r to be increased, leading to greater concern about the lack of the secondary voltage V.sub.2.